Cholinergic neurotransmission, mediated primarily through the neurotransmitter acetylcholine (ACh), is a predominant regulator of the physiological functions of the body via the central and autonomic nervous system. ACh acts on the synapses of the neurons present in of all the autonomic ganglia, neuromuscular junctions and the central nervous system. Two distinct classes of ACh target receptors viz. muscarinic (mAChRs) and the nicotinic (nAChRs) have been identified in brain, forming a significant component of receptors carrying its mnemonic and other vital physiological functions.
Neural nicotinic ACh receptors (NNRs) belong to the class of ligand-gated ion channels (LGIC) comprising of five subunits (α2-α10, β2-β4) arranged in heteropentameric (α4β2) or homopertameric (α7) configuration (Paterson D et al., Prog. Neurobiol., 2000, 61, 75-111). α4β2 and α7 nAChR constitute the predominant subtypes expressed in the mammalian brain. α7 nAChR has attained prominence as a therapeutic target due to its abundant expression in the learning and memory centers of brain, hippocampus and the cerebral cortex (Rubboli F et al., Neurochem. Int., 1994, 25, 69-71). Particularly, α7 nAChR is characterized by a high Ca2+ ion permeability, which is responsible for neurotransmitter release and consequent modulation of excitatory and inhibitory neurotransmission (Alkondon M et al., Eur. J. Pharmacol., 2000, 393, 59-67; Dajas-Bailador F et al., Trends Pharmacol. Sci., 2004, 25, 317-324). Furthermore, high Ca2+ ion influx also has implications on the long-term potentiation of memory via alterations in gene expression (Bitner R S et al., J. Neurosci., 2007, 27, 10578-10587; McKay B E et al., Biochem. Pharmacol., 2007, 74, 1120-1133).
Several recent studies have confirmed the role of α7 nAChR in neural processes like attention, memory and cognition (Mansvelder H D et al., Psychopharmacology (Berl), 2006, 184, 292-305; Chan W K et al., Neuropharmacology, 2007, 52, 1641-1649; Young J W et al., Eur. Neuropsychopharmacol., 2007, 17, 145-155). Gene polymorphisms associated with the α7 nAChR protein CHRNA7 have been implicated in the genetic transmission of schizophrenia, related neurophysiological sensory gating deficits and resultant cognitive impairment (Freedman R et al., Biol. Psychiatry, 1995, 38, 22-33; Tsuang D W et al., Am. J. Med. Genet., 2001, 105, 662-668). Also, preclinical studies in α7 nAChR knock-out and anti-sense oligonucleotide treated mice have demonstrated impaired attention and defective cognition underscoring the prominent role of α7 nAChR in cognition (Curzon P et al., Neurosci. Lett., 2006, 410, 15-19; Young J W et al., Neuropsychopharmacology., 2004, 29, 891-900). Additionally, pharmacological blockade of α7 nAChR impairs memory and its activation enhances same in preclinical rodent models implicating α7 nAChR as target for cognitive enhancement (Hashimoto K et al., Biol. Psychiatry, 2008, 63, 92-97).
Pathological brain function in sensory-deficit disorders has been associated with nicotinic cholinergic transmission particularly through α7 receptors (Freedman R et al., Biol. Psychiatry, 1995, 38, 22-33; Tsuang D W et al., Am. J. Med. Genet., 2001, 105, 662-668; Carson R et al., Neuromolecular, 2008, Med. 10, 377-384; Leonard S et al., Pharmacol. Biochem. Behav., 2001, 70, 561-570; Freedman R et al., Curr. Psychiatry Rep., 2003, 5, 155-161; Cannon T D et al., Curr. Opin. Psychiatry, 2005, 18, 135-140). A defective pre-attention processing of sensory information is understood to be the basis of cognitive fragmentation in schizophrenia and related neuropsychiatric disorders (Leiser S C et al., Pharmacol. Ther., 2009, 122, 302-311). Genetic linkage studies have traced sharing of the α7 gene locus for several affective, attention, anxiety and psychotic disorders (Leonard S et al., Pharmacol. Biochem. Behav., 2001, 70, 561-570; Suemaru K et al., Nippon Yakurigaku Zasshi, 2002, 119, 295-300).
Perturbations in the cholinergic and glutamatergic homeostasis, has long been implicated as causative factors for host of neurological disease, including dementia(s) (Nizri E et al., Drug News Perspect., 2007, 20, 421-429). Dementia is a severe, progressive, multi-factorial cognitive disorder affecting memory, attention, language and problem solving. Nicotinic ACh receptor, particularly the interaction of α7 receptor to αβ1-42 is implicated as an up-stream pathogenic event in Alzheimer's disease, a major causative factor for dementia (Wang H Y et al., J. Neurosci., 2009, 29, 10961-10973). Moreover, gene polymorphisms in CHRNA7 have been implicated in dementia with lewy bodies (DLB) and Pick's disease (Feher A et al., Dement. Geriatr. Cogn. Disord., 2009, 28, 56-62).
Disease modification potential of nAChRs particularly the α7 receptor has application for disease-modification of Alzheimer's disease (AD) and Parkinson's disease (PD) by enhancing neuron survival and preventing neurodegeneration (Wang et al. 2009; Nagele R G et al., Neuroscience, 2002, 110, 199-211; Jeyarasasingam G et al., Neuroscience, 2002, 109, 275-285). Additionally, α7 nAChR induced activation of anti-apoptotic (BCL-2) and anti-inflammatory pathways in brain could have neuroprotective effects in neurodegenerative diseases (Marrero M B et al., Brain. Res., 2009, 1256, 1-7). Dopamine containing neurons of ventral tegmental area (VTA) and laterodorsal tegmental nucleus (LDT) are known to express nicotinic ACh receptors, particularly α4, α3, β2, β3, β4 subunits (Kuzmin A et al., Psychopharmacology (Berl), 2009, 203, 99-108). Nicotinic ACh receptors, α4β2 and α3β4 have been identified with candidate-gene approach to have strong mechanistic link for nicotine addiction (Weiss R B et al., PLoS Genet., 2008, 4, e1000125). α7 nAChR has particularly been studied for a putative role in cannabis addiction (Solinas M et al., J. Neurosci., 2007, 27, 5615-5620). Varenicline, a partial agonist at α4β2, has demonstrated better efficacy in reducing the smoking addiction and relapse prevention in comparison to buproprion (Ebbert J O et al., Patient. Prefer. Adherence, 2010, 4, 355-362).
Presence of a high-affinity nicotine binding site at α4β2 nAChR, in the descending inhibitory pathways from brainstem has sparked interest in the antinociceptive properties of nicotinic ACh receptor agonists like epibatidine (Decker M W et al., Expert. Opin. Investig. Drugs, 2001, 10, 1819-1830). Several new developments have opened the area for use of nicotinic modulators for therapy of pain (Rowbotham M C et al., Pain, 2009, 146, 245-252). Appropriate modulation of the nicotinic ACh receptors could provide for remedial approach to pain related states.
Another key role of the α7 nAChR is the ability to modulate the production of pro-inflammatory cytokines, like interleukins (IL), tumor necrosis factor alpha (TNF-α), and high mobility group box (HMGB-1) in the central nervous system. Consequently, an anti-inflammatory and antinociceptive effect in pain disorders have been demonstrated (Damaj M I et al., Neuropharmacology, 2000, 39, 2785-2791). Additionally, ‘cholinergic anti-inflammatory pathway’ is proposed to be a regulatory of local and systemic inflammation and neuro-immune interactions through neural and humoral pathways (Gallowitsch-Puerta M et al., Life Sci., 2007, 80, 2325-2329; Gallowitsch-Puerta and Pavlov 2007; Rosas-Ballina M et al., Mol. Med., 2009, 15, 195-202; Rosas-Ballina M et al., J. Intern. Med., 2009, 265, 663-679). Selective modulators of nicotinic ACh receptors, particularly α7 type, like GTS-21, attenuate cytokine production and IL-1β after endotoxin exposure. Furthermore, α7 nAChR are understood to have a central role in arthritis pathogenesis and potential therapeutic strategy for treatment of joint inflammation (Westman M et al., Scand. J. Immunol., 2009, 70, 136-140). A putative role for α7 nAChR has also been implicated in severe sepsis, endotoxemic shock and systemic inflammation (Jin Y et al. (2010) Int. J. Immunogenet., Liu C et al., Crit. Care. Med., 2009, 37, 634-641).
Angiogenesis, is a critical physiological process for the cell survival and pathologically important for cancer proliferation; several non-neural nicotinic ACh receptors, particularly α7, α5, α3, β2, β4, are involved (Arias H R et al., Int. J. Biochem. Cell. Biol., 2009, 41, 1441-1451; Heeschen C et al., J. Clin. Invest., 2002, 110, 527-536). A role of nicotinic ACh receptors in the development of cervical cancer, lung carcinogenesis and paediatric lung disorders in smoking-exposed population has also been studied (Calleja-Macias I E et al., Int. J. Cancer., 2009, 124, 1090-1096; Schuller H M et al., Eur. J. Pharmacol., 2000, 393, 265-277). Several α7 nAChR agonists, partial agonists, have been characterized for their efficacy in clinical and preclinical studies. EVP-6124, an agonist at α7 nAChR, has demonstrated significant improvement in sensory processing and cognition biomarkers in Phase Ib study with patients suffering from schizophrenia (EnVivo Pharmaceuticals press release 2009, Jan. 12). GTS-21 (DMXB-Anabaseine), an α7 nAChR agonist, in the P II clinical trials, has shown efficacy in improving cognitive deficits in schizophrenia and inhibition of endotoxin-induced TNF-α release (Olincy A et al., Biol. Psychiatry, 2005, 57(8, Suppl.), Abst 44; Olincy A et al., Arch. Gen. Psychiatry, 2006, 63, 630-638; Goldstein R et al., Acad. Emerg. Med., 2007, 14 (15, Suppl. 1), Abst 474). CP-810123, a α7 nAChR agonist, exhibits protection against the scopolamine-induced dementia and inhibition of amphetamine-induced auditory evoked potentials in preclinical studies (O'Donnell C J et al., J. Med. Chem., 2010, 53, 1222-1237). SSR-180711A, also an α7 nAChR agonist, enhances learning and memory, and protects against MK-801/Scopolamine-induced memory loss and prepulse inhibition in preclinical studies (Redrobe J P et al., Eur. J. Pharmacol., 2009, 602, 58-65; Dunlop J et al., J. Pharmacol. Exp. Ther., 2009, 328, 766-776; Pichat P et al., Neuropsychopharmacology, 2007, 32, 17-34). SEN-12333, protected against scopolamine-induced amnesia in passive avoidance test in preclinical studies (Roncarati R et al., J. Pharmacol. Exp. Ther., 2009, 329, 459-468). AR-R-17779, an agonist at α7 nAChR, exhibits improvement in the social recognition task performed in rats (Van K M et al., Psychopharmacology (Berl), 2004, 172, 375-383). ABBF, an agonist at α7 nAChR, improves social recognition memory and working memory in Morris maze task in rats (Boess F G et al., J. Pharmacol. Exp. Ther., 2007, 321, 716-725). TC-5619, a selective α7 nAChR agonist has demonstrated efficacy in animal models of positive and negative symptoms and cognitive dysfunction in schizophrenia (Hauser T A et al., Biochem. Pharmacol., 2009, 78, 803-812).
An alternative strategy to reinforce or potentiate the endogenous cholinergic neurotransmission of ACh without directly stimulating the target receptor is the positive allosteric modulation (PAM) of α7 nAChR (Albuquerque E X et al., Alzheimer Dis. Assoc. Disord., 2001, 15 Suppl 1, S19-S25). Several PAMs have been characterized, albeit in the preclinical stages of discovery. A-86774, α7 nAChR PAM, improves sensory gating in DBA/2 mice by significantly reducing the T:C ratio in a preclinical model of schizophrenia (Faghih R et al., J. Med. Chem., 2009, 52, 3377-3384). XY-4083, an α7 nAChR PAM, normalizes the sensorimotor gating deficits in the DBA/2 mice and memory acquisition in 8-arm radial maze without altering the receptor desensitization kinetics (Ng H J et al., Proc. Natl. Acad. Sci., U.S.A., 2007, 104, 8059-8064). Yet another PAM, PNU-120596, profoundly alters α7 nAChR desensitization kinetics and simultaneously protecting against the disruption of prepulse inhibition by MK-801. NS-1738, another PAM, has exhibited efficacy in-vivo in the animal models of social recognition and spatial memory acquisition in the Morris maze task (Timmermann D B et al., J. Pharmacol. Exp. Ther., 2007, 323, 294-307). In addition, several patents/applications published are listed below—US20060142349, US20070142450, US20090253691, WO2007031440, WO2009115547, WO2009135944, WO2009127678, WO2009127679, WO2009043780, WO2009043784, U.S. Pat. No. 7,683,084, U.S. Pat. No. 7,741,364, WO2009145996, US20100240707, WO2011064288, US20100222398, US20100227869, EP1866314, WO2010130768, WO2011036167, US20100190819 disclose efficacy of allosteric modulators of nicotinic ACh receptors and underscoring their therapeutic potential.